1. Technical Field
The embodiments described herein relate to a semiconductor integrated circuit, and more particularly, to an apparatus for adjusting the resistance value of a driver in a semiconductor integrated circuit.
2. Related Art
In conventional semiconductor integrated circuits, the resistance value of a data output driver should be stably maintained at a target value in order to properly output data. A periodic adjusting process is needed to maintain the resistance value of the driver at a constant value regardless of an external environment, such as voltage or temperature.
A conventional process of adjusting the resistance value of the driver uses an external resistor RZQ that is connected to the semiconductor integrated circuit through a pin (hereinafter, referred as a ZQ pin) in order to set up an exact standard.
The process of adjusting the resistance value of the driver using the external resistor RZQ is performed as follows: A digital-to-analog converting circuit connected to the external resistor RZQ converts a predetermined code value into an analog voltage, and the analog voltage is compared with a reference voltage. The digital-to-analog converting circuit and the driver are similarly modeled. The predetermined code value is changed according to the result of the comparison and the comparing process is repeated until the analog voltage is equal to the reference voltage. When the analog voltage is equal to the reference voltage, the code value is input to the driver to adjust the resistance value of the driver.
A conventional semiconductor integrated circuit has a fixed adjustable range of the resistance value according to a variation in the code value, that is, a fixed resolution in adjusting the resistance value of the driver. As shown in FIG. 1, a conventional semiconductor integrated circuit increases or decreases the resistance value at the fixed resolution according to the result obtained by the comparing process to change the voltage, and compares the changed voltage with the reference voltage, thereby adjusting the resistance value of the driver.
Since the resolution is fixed during the process of adjusting the resistance value of the driver, it takes a lot of time to adjust the resistance value and the adjustable range of the resistance value is limited.